In an autonomous mobile system such as a robot or an automatic guided vehicle, it is necessary to prevent collision and falling by detecting an obstacle or a level difference which is in front during movement. For such detection of an obstacle or a level difference, a surrounding environment recognition device using a sensor is being developed.
As a sensor which is used for such applications, a laser range finder (LRF) and a distance image sensor exist.
The laser range finder has a structure of performing angle scanning while projecting laser light to a measurement target and detecting reflection light therefrom to thereby measure a distance to the target. Though relatively high measurement accuracy is achieved, as the angle scanning is normally for one direction, acquired with single measurement is only distribution information of a one-dimensional distance. In order to acquire distribution of a two-dimensional distance, contrivances of performing measurement by arranging a plurality of laser range finders in parallel, performing measurement a plurality of times with mechanical movement, and the like are required.
As the distance image sensor, some types including a pattern light irradiation type, a time Of flight (TOF) type, and a stereo type exist according to a method for measuring a distance, but all of them are characterized by outputting a distance image in which distance information of a measurement target within a visual field range of an imaging element is represented by density of a pixel, making it possible to acquire two-dimensional distance information at a time with single measurement. Accordingly, it is suitable for using as a sensor of a surrounding environment recognition device which acquires a distance to a circumferentially existing object promptly and recognizes whether it is an obstacle or a level difference.
In an autonomous mobile system shown in PTL 1 (Japanese Unexamined Patent Application Publication No. 2006-260105), a distance image sensor which is attached so as to have a visual field range as a frontward lower side of an advancing direction is used to detect presence/absence of a convex obstacle and a concave level difference which exist on a frontward travelling surface. Further, the distance image is subjected to coordinate conversion, which is then binarized with two different thresholds, to thereby judge whether shapes of the obstacle and the level difference are slope shapes or step shapes.